A pest is "a plant or animal detrimental to humans or human concerns (as agriculture or livestock production)";[1] alternative meanings include organisms that cause nuisance and epidemic disease associated with high mortality (specifically: plague). In its broadest sense, a pest is a competitor of humanity.[2]

A pest is any living organism which is invasive or prolific, detrimental, troublesome, noxious, destructive, a nuisance to either plants or animals, human or human concerns, livestock, or human structures. It is a loose concept, as an organism can be a pest in one setting but beneficial, domesticated or acceptable in another.

Pests often occur in high densities, making the damage they do even more detrimental. Termite

The term pest may be used to refer specifically to harmful animals but is also often taken to mean all harmful organisms including weeds, plant pathogenic fungi and viruses. Pesticides are chemicals and other agents (e.g. beneficial micro-organisms) that are used to control or protect other organisms from pests. The related term vermin has much overlap with pest, but generally only includes those creatures that are seen to be vectors of diseases.

It is possible for an animal to be a pest in one setting but beneficial or domesticated in another (for example, European rabbits introduced to Australia caused ecologicaldamage beyond the scale they inflicted in their natural habitat). Many weeds are also seen as useful under certain conditions, for instance Patterson's curse is often valued as food for honeybees and as a wildflower, even though it can poison livestock.

The term "plant pest" has a very specific definition in terms of the International Plant Protection Convention and phytosanitary measures worldwide. A pest is any species, strain or biotype of plant, animal, or pathogenic agent injurious to plants or plant products.[4]

Plants may be considered pests themselves if an invasive species. Any prolific animal or plant may be considered pests.

Thus in contrast, to the single term "pest", in composite terms, e.g. "plant pest", "agricultural pest" "livestock pest", the emphasis is always on the organisms or ecosystems infested and affected. This distinction is followed below.

Cane toads have had serious negative effects on many ecosystems to which they have been introduced, especially in Australia. The toad's skin is toxic, killing many wild and domestic animals that attempt to eat it.

^Lowe S., Browne M., Boudjelas S. and de Poorter M. (2000). 100 of the World’s Worst Invasive Alien Species. A selection from the Global Invasive Species Database. The Invasive Species Specialist Group (ISSG), a specialist group of the Species Survival Commission (SSC) of the World Conservation Union (IUCN), Auckland.